1. Field of the Invention
The present invention relates to a finder display apparatus, and more specifically, to a finder display apparatus for use in a finder optical system of a camera.
2. Description of the Prior Art
Methods and apparatuses have been proposed for displaying various kinds of information necessary for photographing within a finder field of a camera. The information necessary for photographing includes information on a shutter speed and an aperture value associated with exposure, information on a photographic area (field frame) associated with the determination of a photo composition which is an important function of a finder, a focusing area (distance measurement frame) and a photometric area (photometric frame), information on a result of focusing (in/out of focus, front/rear focus) and a direction of focusing, and information associated with flash light. In many cases, these pieces of information vary with photographing occasions. Therefore, it would be convenient for the user looking through the finder in order to perform photographing if, of these pieces of information, only necessary ones are selectively displayed within the finder field according to circumstances. As effective means for realizing this, finder display apparatuses where an electro-optical device such as liquid crystal is used as a display device are known (e.g. the ones disclosed by Japanese Laid-open Utility Model Application No. S60-92234, Japanese Laid-open Patent Applications Nos. S52-110626, S57-204028, S58-85426, S58-13625 and S62-272236).
Conventional finder display apparatuses for use in finder optical systems will hereinafter be described where information as well as a finder image viewed through an eyepiece provided in the finder optical system is displayed within the finder field by means of an electro-optical device constituted by twisted nematic liquid crystal (hereinafter referred to as TN liquid crystal) sandwitched between two polarizing members.
FIG. 2 is a longitudinal cross-sectional view schematically showing a finder optical system for use in a single-lens reflex camera where a finder display apparatus which is a first prior art is employed. This prior art is a finder display apparatus for "within-image-plane display" (explained later) where an electro-optical device requiring two polarizing plates is used as a display device.
An example of the finder display is a so-called superimposed finder display which is displayed in the entire area or an arbitrary area of an image plane within the finder field for viewing a subject image, being superimposed on the subject image (this type of the finder display will hereinafter be referred to as the "within-image-plane display"). Hereinafter, "image plane" will indicate an area of a subject image obtained through a finder optical system unless otherwise indicated. Thus, a finder field (image) viewed by the user through the finder includes an image within the "image plane" and an image outside the "image plane." A type of the finder display where necessary information is displayed outside the image plane within the finder field will be referred to as "outside-image-plane display."
In FIG. 2, 1 is a main mirror, 2 is a focusing screen, 4 is a pentaprism, 5 is an eyepiece system, 6 is a photometric lens system, 7 is a photometric light receiving device, 9 is a sub mirror, 10 is a focusing optical system, 11 is a focusing light receiving device, EP is a pupil, and 301 is an electro-optical display device typically requiring two polarizing plates. The electro-optical display device 301 is a TN liquid crystal device constituted by TN liquid crystal.
At the front and rear of the electro-optical display device 301 along an optical axis AX, polarizing plates 81 and 82 are arranged under a condition where they are attached to base plates 32 and 36 constituting the electro-optical display device 301. The structure and display principle of the TN liquid crystal device will be described later with reference to FIG. 1. The combination of polarization directions of the polarizing plates 81 and 82 is univocally determined depending on the orientation (or rotary polarization) of the TN liquid crystal constituting the electro-optical display device 301. TN liquid crystal has generally been used for such display devices since it is advantageous in cost, space and information amount.
As shown in FIG. 2, light from a subject having passed through a non-illustrated taking lens is partly reflected upward at 90.degree. by a main mirror 1 which is a semitransparent mirror, and reaches a focusing screen 2 to form on the matt 21 a real image approximately equivalent to an image formed on film. In the case where a condenser lens or a Fresnel lens is arranged in front of the matt 21, the image formed on the matt 21 is slightly smaller than the image formed on film. Of the subject light reaching the main mirror 1, the remaining portion of the light which is not reflected but passes through the main mirror 1 is, after having passed through the main mirror 1, directed through the focusing optical system 10 to the focusing light receiving device 11 for automatic focusing. Focusing is performed by use of an output from the focusing light receiving device 11.
In order to turn the real image formed on the matt 21 right way around to obtain an erecting finder image (virtual image), the pentaprism 4 is arranged above the focusing screen 2. In order to enlarge the real image formed on the matt 21 to obtain a virtual image at an appropriate position, the eyepiece system 5 is arranged between the pentaprism 4 and the pupil EP. Between the focusing screen 2 and the pentaprism 4, the electro-optical display device 301, sandwitched between the polarizing plates 81 and 82, is arranged. The polarizing plates 81 and 82 are respectively attached to the base plates 32 and 36 constituting the electro-optical display device 301.
The structure of the TN liquid crystal constituting the electro-optical display device 301 used in the first prior art will briefly be described. FIG. 1 schematically shows a cross section of a typical TN liquid crystal device. In the figure, 31 and 37 are polarizing plates, 32 and 36 are base plates made of glass or plastic, 33a, 33b, 33c and 35 are transparent electrodes, 34 is liquid crystal, 38 is a sealing member. In the figure, leads for supplying power and molecular orientation films on the liquid crystal side surface of the transparent electrodes 33a, 33b, 33c and 35 are not shown.
The combination of polarization directions of the polarizing plates 31 and 37 is determined according to the orientation (or rotary polarization) of liquid crystal molecules in the liquid crystal 34 filled between the base plates 32 and 36. A typical combination is the one where the polarization directions of the two polarizing plates 31 and 37 are perpendicular to each other.
Subsequently, the display principle of the TN liquid crystal device of FIG. 1 will be described. Considering now a liquid crystal display device whose entire surface is transparent (i.e. so-called positive display), when no voltage is applied, if the polarization directions of the two polarizing plates 31 and 37 are perpendicular to each other as mentioned above, the orientation of the liquid crystal molecules in the liquid crystal 34 is twisted by 90.degree.. For this reason, when no voltage is applied, light incident on the liquid crystal device, for example, from the polarizing plate 31 side has only linearly polarized components after passing through the polarizing plate 31. The polarization direction of the light having only linearly polarized components is, although the linearly polarized condition is maintained, rotated by 90.degree. by the influence of the orientation of the liquid crystal molecules. The light whose polarization direction has been rotated is incident on the base plate 36 and passes therethrough. Then, lastly, the light again passes through the polarizing plate 37. Since the polarization direction of the polarizing plate 37 is perpendicular to that of the polarizing plate 31, if the conversion of the polarization direction by the liquid crystal is ideally made, the quantity of the light incident on the polarizing plate 31 and that of the light exiting from the polarizing plate 37 are approximately the same. Thus, the liquid crystal device is transparent, that is, transmits light when no voltage is applied thereto.
Subsequently, an operation of the liquid crystal device when a voltage is applied thereto will be described. Of the transparent electrodes 33a, 33b, 33c and 35 shown in FIG. 1, the transparent electrodes 33a to 33c are arranged in areas covering the shapes of a plurality of kinds of information displayed when the liquid crystal device is used as a display device, while the transparent electrode 35 is arranged to cover nearly an entire surface of the liquid crystal 34.
For example, when a voltage is applied only to the transparent electrode 33a and the transparent electrode 35, only liquid crystal molecules present in a portion sandwitched between the transparent electrodes 33a and 35 are oriented. Specifically, they are oriented so that their lengths are perpendicular to the base plates. Consequently, the polarization direction of light passing through this portion is not rotated. As a result, the light passing through this portion passes through the liquid crystal 34 and the glass plate 36 without its polarization condition being changed.
However, since the polarization direction (polarization plane of transmitted light) of the polarizing plate 37 is perpendicular to the polarization direction of the polarizing plate 31, the light having passed through the portion of the liquid crystal 34 which is sandwitched between the transparent electrodes 33a and 35 does not pass through the polarizing plate 37 after passing through the base plate 36. That is, the area corresponding to the shape formed by the transparent electrode 33a has no light impermeability. Since the portions of the other transparent electrodes (the portions of the transparent electrodes 33b and 33c in FIG. 1) transmit light as described above, only the area corresponding to the shape (i.e. display pattern) formed by the transparent electrode 33a appears dark since it does not transmit light. The function as a display device is thus achieved.
With the feature of the first prior art, the user can simultaneously observe an information display and a subject image on the matt 21 by displaying necessary information on the electro-optical display device 3 by using a display controlling circuit and a driving circuit (not shown).
FIG. 3 is a longitudinal cross-sectional view schematically showing a finder optical system for use in a single-lens reflex camera where a finder display apparatus which is a second prior art is shown. This prior art is a finder display apparatus where an electro-optical device requiring two polarizing plates is used as a display device.
In FIG. 3 the same elements as those of the above-described first prior art of FIG. 2 are denoted by the same reference designations. In FIG. 3, 81a and 82a are polarizing plates respectively attached to base plates 32a and 36a constituting an electro-optical display device 302, 12 is a light source for lighting the electro-optical display device 302 from the rear side thereof, and 13 is a display prism for directing toward the pupil EP a luminous flux coming from the light source 12 and the electro-optical display device 302, which luminous flux is associated with display.
With the feature of the second prior art, by displaying necessary information on the electro-optical display device 302 by using a display controlling circuit and a driving circuit (not shown), the luminous flux transmitted by the electro-optical display device 302 passes through the display prism 13, the pentaprism 4 and the eyepiece system 5, so that the information display can be viewed at the pupil EP. At this time, the information display is located outside the image plane within the finder field and does not interfere in the subject image.
FIG. 4 is a longitudinal cross-sectional view schematically showing a finder optical system for use in a single-lens reflex camera where a finder display apparatus which is a third prior art is employed. This prior art is a finder display apparatus for both the within-image-plane and outside-image-plane displays where an electro-optical device requiring two polarizing plates is used as a display device. In FIG. 4, the same elements as those of the above-described first and second prior arts of FIGS. 2 and 3 are denoted by the same reference designations. The elements the same as those of the second prior art are arranged at different positions.
Of the information displays to be displayed within the finder field, some are of relatively simple shape (e.g. a field frame) and others are of fine shape (e.g. a numeral and a small symbol typically provided by means of segment display). It is not very easy to form displays of different finenesses on one electro-optical device. For example, in the case of an electro-optical device constituted by TN liquid crystal, it is sometimes difficult to form on one device a plurality of kinds of displays requiring different positioning accuracies of two opposing electrodes. In such a case, the cost is lower if displays of fine shape and displays of not fine shape are separately formed on two different electro-optical devices. The third prior art shown in FIG. 4 is an example of a finder display apparatus suitable for such a case.
However, the above-described prior arts present the following problems.
The first problem is that the quality of the finder image deteriorates due to flaws or dirt on the polarizing plate. The polarizing plates readily get flawed and dirty. Since the polarizing plates are arranged in the vicinity of the focusing screen, the flaws or the dirt is readily viewed when the finder field is observed. As a result, the performance of the finder optical system deteriorates because of its poor image quality.
For example, in the case where the electro-optical display device 301 and the polarizing plates 81 and 82 are arranged in the vicinity of an image plane (matt 21) formed by an objective lens system (including a taking lens system associated with a finder optical system for a single-lens reflex camera as well as an objective lens system in a real-image-type finder optical system) like the first and third prior arts, if the polarizing plates 81 and 82 are flawed or dirty, the flaws or the dirt in the vicinity of the focusing screen is clearly viewed since the eyepiece is arranged so that the dioptric power is adjusted with respect to the focusing screen. As a result, the quality of the finder image deteriorates. Regarding the polarizing plates 81a and 82a of the second and third prior arts for the outside-image-plane display, since they are arranged in the vicinity of a position corresponding to the image plane on the optical path, i.e. a position with respect to which the dioptric power is adjusted by the eyepiece, if they are flawed or dirty, the finder image quality similarly deteriorates. Since it is necessary to consider the prevention of the flaws and dirt on the polarizing plates in order to prevent the deterioration of the finder image quality, the assembling must be performed more carefully, which will lead an increase in cost.
The second problem is that the polarizing plate exerts a bad influence on light for photometry. In the case where photometry for determining exposure is performed by using a part or the whole of a finder luminous flux, if a luminous flux having passed through the polarizing plate is used for the photometry, an output value of the photometric light receiving device varies according to the polarization condition of light from a subject. As a result, exposure can be adversely affected. Moreover, since the quantity of light reaching the photometric light receiving device decreases, the low luminance limit performance in photometry deteriorates. For example, in the first and third prior arts of FIGS. 2 and 4, in order to cope with the within-image-plane display, the polarizing plates 81 and 82 are arranged to cover nearly an entire surface of the focusing screen 2. For this reason, if light having passed through the focusing screen 2 is used for photometry, a photometric value can be affected according to the polarization condition of the light from a subject. As a result, it is difficult to automatically obtain an appropriate exposure.
The third problem is that the dioptric power of the display and that of the subject image differ. Since polarizing plates are, typically, 0.3 mm thick, it cannot be helped that the electro-optical display device is away from the focusing screen at least by the amount of the thickness of the polarizing plate. For this reason, for example, in the case of a finder optical system like the first prior art where a subject image and a finder display are simultaneously viewed, the dioptric power of the subject image and that of the finder display differ. Since the larger the difference is, the poorer the finder image quality is, it is preferable to arrange the display optical device to be as close to the focusing screen as possible.
The fourth problem is that it is necessary to use large polarizing plates. In the case where information is displayed within the image plane in the finder field by using an electro-optical device, it is essential that the displayed information should easily be viewed. For this reason, in order to obtain an appropriate finder dioptric power, the electro-optical device is arranged in the vicinity of an image plane formed by the objective lens provided in the finder optical system. Therefore, it has been required that the polarizing plates attached to the base plates of the electro-optical device should be approximately as large as the image formed plane. The use of such large polarizing plates increases the cost.
The fifth problem is that a space produced due to the disposition of the electro-optical display device increases the size of the apparatus. Specifically, the disposition of the electro-optical display device is limited by the thickness of the polarizing plates. Moreover, the fact that the electro-optical display device to which the polarizing plates are attached must be held by some means further limits the disposition of the electro-optical display device, since the polarizing plates must not be directly touched and must not be pressurized.